Thermally processed steel

ABSTRACT

This invention relates to a process for producing an austenitic steel with a combination of high strength and high elongation by a process involving thermal manipulation. No mechanical deformation is essential but can contribute some additional benefits. The process taught by this invention is applicable to any steel in which the Ms is well below the lowest anticipated service temperature, the austenite reversion temperature is substantially below the point at which the austenite recrystallizes, and where the Md is increased by the thermal manipulation. This invention involves repeated thermal cycling between austenite and martensite.

United States Patent 1111 Koppenaal et a1.

1451 Nov. 18,1975

1 1 THERMALLY PROCESSED STEEL {75] Inventors: Theodore J. Koppenaal, Newport Beach; Richard P. Sernka, Irvine, both of Calif. *[73] Assignee: Ford Motor Company, Dearbom,

' Mich. [22] Filed: Dec. 7, 1973 [21] Appl. No.: 422,947

Related US. Application Data [63] Continuation-impart of Ser. No. 172,808, Aug. 8,

1971, abandoned.

[52] US. Cl. l48/l2.4; 148/125; 148/143; v 148/144 [51] Int. Cl. C2lD 6/04; C2lD 7/02; C2lD 1/22 [58] Field of Search 148/2, 3, 12, 12.4, 13, l48/14,-l34,143,144,12.3,125

References Cited UNITED STATES PATENTS 3,288,657 11/1966 Morita 61211. 148/143 3,316,129 4/1967 Token et al 148/135 3,370,994 2/1968 Konkol 148/142 3,378,367 4/1968 F1115 et a1 75/128 3,385,740 5/1968 Baggstrom et al. 148/136 3,488,231 l/l970 Zackay et a1. 148/12 OTHER PUBLICATIONS Transactions of the Metallurgical Society of AIME, Vol. 224, Dec. 1962-, pp. 1212-4221.

Transactions of ASM, Vol. 61, 1968, pp. 26-36.

Primary Examiner-C. Lovell Attorney, Agent, or Firm.loseph W. Malleck; Keith L. Zerschling [57] ABSTRACT This invention relates to a process for producing an austenitic steel with a combination of high strength and high elongation by a process involving thermal manipulation. No mechanical deformation is essential but can contribute some additional benefits. The process taught by this invention is applicable to any steel 'in which the M is well below the lowest anticipated service temperature, the austenite reversion temperature is substantially below the point at which the austenite recrystallizes, and where the M is increased by the thermal manipulation. This invention involves repeated thermal cycling between austenite and martensite.

7 Claims, 12 Drawing Figures US. Patent Nov. 18, 1975 Sheet 3 of7 3,920,490

1 NVENTORS E/(HAED d'fE/VKA 7000 1 KOP/f/VAAL ATTORNEYS FIG INVENTORS A Jff/VKA JXfiFPf/VAAL ATTORNEYS U.S. Patent Nov. 18, 1975 INTENSITY Sheet 5 of 7 FIGJO l /\AFTER FIFTH REVERSION /\A=TER FlksT REV ERSION AS QUE NCHED I30 I29 I28 I27 I26 I25 26, DEGREES INVENTORS 4 /664450 556/1664 7791 05061? J KOPPf/Vfl/M ATTORNEYS THERMALLY PROCESSED STEEL CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of our copending application Ser. No. 172,808, filed Aug. 8, I971, now abandoned and having the same title as the present application.

BACKGROUND OF THE INVENTION The current invention permits the production of austenitic steels having a combination of high strength and high elongation without the necessity of deforming the steel to obtain these properties. Zackay and Parker recently were awarded U.S. Pat. No. 3,488,231, Jan. 6, 1970, for developing a class of high strength and high elongation steels wherein these mechanical properties are achieved by the use of a deformation cycle during the processing. This prior art method can best be characterized as a warm plastic deformation cycle wherein the austenite is strained by reduction in thickness up to 80%. and by working at temperatures in the range of 800l200F; the yield strength can be increased up to about 200 ksi by this method. Since the high elongation values resulted from the control of the austenite to martensite phase transformation, Zackay and Parker labeled the phenomenon TRIP, which is abbreviated from the words Transformation Induced Plasticity.

This invention provides an alternative to this prior art method by avoiding the requirement for large equip ment needed to impose strain and by avoiding the limitation to thin sections. Thus, under ambient temperature service conditions, there will be a transformation from strained austenite to martensite rendering an increase in strength and ductility for the particular steel. but this will have been brought about by a unique thermal cycling, rather than by mechanical treatment.

According to Zackay and Parker and other related prior art, the thermal martensite start temperature, M must be below ambient while only the deformation or martensite start temperature, M,,, is above ambient. Warm plastic deformation will then harden the austenite and reportedly promotes carbide precipitation, thus altering the matrix chemistry. This combination appropriately adjusts the M, and M,, with the ambient temperature condition occurring between them. When alloys, so processed, are stressed at room temperature, the onset of plastic strain is accompanied by transformation from cold worked austenite to martensite; the amount (volume) transformed being partially dependent upon the amount of plastic strain. This transformation to martensite strengthens the mass locally, shifting the region of strain. This has the effect of producing extensive uniform strain (elongation) in the volumes stressed in tension. Strength is primarily increased by cold working the austenite and the high elongation is a result of the shift in M to above room temperature.

In contrast, this invention uses steels which have the starting M, and M,, both below ambient or room temperatures; but, surprisingly, the method herein permits the adjustment of the M, and M solely by thermal excursions so that the ambient temperature will similarly lay between them. The shift in the M temperature is the primary reason for the ability to obtain high elongation at high tensile strength values.

SUMMARY OF THE INVENTION A primary object of this invention is to obtain a combination of high strength and high ductility in TRIP type steels essentially without the need for special prior mechanical deformation as required in US. Pat. No. 3,488,231.

Another object is to provide a method for raising the M, of a substantially austenitic steel without the use of mechanical deformation and thereby provide for transformation of austenite to martensite under ambient temperature service conditions, resulting in a combination of greater strength and ductility.

Still another object of this invention is to provide a method for making transformation induced plasticity steels which can be welded without destroying the effects of the processing, contrary to thermomechanically processed steels, which can be used in castings of intricate shapes, and which will not necessarily be highly anisotropic.

BRIEF SUMMARY OF THE DRAWINGS FIG. 1 is a graph in which the ordinates are R, hardness and the abscissae are the temperature at which the mixture of martensite and austenite was heated to cause reversion. The two curves represent two steels which contain 24% nickel and 4% molybdenum. The first steel contains 0.29% carbon and the other steel 0.22% carbon. The designation on the drawing alpha prime refers to martensite and the designation gamma indicates austenite in conformance with usual metallurgical practice;

FIG. 2 is an outline of the R hardness obtained in steel amenable to the procedure of this invention after five excursions through the thermal cycle outlined above. The martensite was formed at 320F and the austenite by a 2 minute immersion in a salt bath at 1300F;

FIG. 3 is similar to FIG. 2 except that it illustrates properties obtained in a sample A of an inch thick and a sample 0.060 inch thick. The reason for this difference between samples of varying thicknesses is not currently understood;

FIG. 4 is a photomicrograph at a magnification of X showing the steel as quenched from l900F. This structure is entirely austenite and has a grain size of ASTM 0;

FIG. 5 demonstrates the structure obtained after cooling to 320F in liquid nitrogen and at a magnification of X200;

FIG. 6 shows the same steel after being reverted for 2 minutes in a salt bath at l300F. The sample is in excess of 99% austenite and the magnification is X200;

FIG. 7 shows the same steel after a second cooling to 320F following a previous reversion cycle. Martensite, reverted austenite and virgin austenite are observable. The magnification again is X200;

FIG. 8 shows the final structure obtained after five reversion cycles. The magnification is again X200;

FIG. 9 has been presented to show the structure after annealing at 1900F for 30 minutes of a sample previously cycled through five reversions. This sample is not typical of the invention. The magnification is again X200;

FIG. 10 has been presented to show the intensity vs. 26 diffraction angle of (220) austenite peaks for the steel in the as-quenched condition, after a single reversion at l300F and after the fifth reversion at l300F;

FIG. 11 represents the stress and plastic strain of a suitable steel after various thermal processes. This drawing clearly shows the improvement in the steel as the number of reversions is increased;

FIG. 12 demonstrates the physical properties obtained after five reversions and clearly demonstrates the variation of physical properties as influenced by the reversion temperatures;

Table I is a table listing investigative results for a series of alloys from which the scope of the improvement can be proven and predicted. and

Table II is a tabulation of mechanical properties varying with thermal cycling.

DETAILED DESCRIPTION A selected alloy is heated to produce a recrystallized austenite phase. then subjected to a cooling treatment to below ambient temperatures (preferably about 320F) where about 70% of the austenite will be transformed to martensite. The selected alloy is then reheated to a temperature just above that where martensite begins to revert to austenite, but below the temperature for recrystallization into austenite. At this second stage, there is approximately 25% original austenite, 2-3% of the martensite and about 72% reverted austenite. The alloy may then be cooled to room temperature or the same thermal cycle may be repeated preferably at or above five times. The resulting steel will be substantially austenitic which transforms to martensite upon strain from a test device or by use. This method makes it possible for the first time, for an unworked austenite to be capable of transforming to martensite under such conditions.

To practice the invention successfully, it is necessary that-the selected alloy meet several requirements. The steel must have an M,- below ambient temperature. The M, is a standard metallurgical term and is defined as the temperature at which the martensite phase begins to form as the temperature is lowered. The M,, ofa specimen may readily be measured by routine tests that are familiar to those skilled in the art. The M of the starting material must preferably be below ambient temperature. The M is a standard metallurgical term defined as the temperature above which the martensite phase cannot form during working of the metal. The M may readily be determined by standardized testing familiar to those skilled in the art. It is essential that the thermal cycling raise the M,, temperature above the expected service temperature. Lastly, the starting material must also have an austenite reversion temperature below the austenite recrystallization temperature.

The selected steel alloys used in the present investigation, which proved to be successful, substantially contained between l9.924.2% nickel, 2.034.l% molybdenum, l.85% chromium, between 0.22 and 0.36% carbon, and the balance iron. However, the investigation covered alloys having an analysis range of I5.9-24.2% nickel, 2.0l-6.08% molybdenum, 06.06% chromium, 0-l .04% manganese, 00.24% silicon, 0-O.97% cobalt, 0.22-0.36% carbon and the balance iron.

Turning now in some detail, the selected alloy steel is (A) normally received in the completely austenitic condition or may be processed into this condition by heating to a temperature on the order of 1800F to 2200F followed by a water quench. Alloy steels so quenched are essentially 100% austenitic and are relatively soft and ductile in this condition. For example, an

alloy steel having 24% nickel, 4% molybdenum, 0.2-0.3% carbon, balance Fe, had the following mechanical properties: 42,600 psi yield strength, 89,300 psi tensile strength, 38 elongation, and R (about 0 R hardness. The structure of these steels remains austenitic during tensile testing indicating that the M,, is below ambient temperatures.

(B) The steel is next cooled to a temperature below M,,; in the case of the example. the steel was cooled in liquid nitrogen to 320F. The steel is then allowed to warm to ambient temperature. The steel is now a mixture of martensite and austenite and the example had the following mechanical properties: 1 19,600 psi yield strength, 155,900 tensile strength, 13% elongation and 31 R hardness.

(C) The steel is then immersed in a liquid salt bath at a temperature above the austenite reversion temperature but below the austenite recrystallization temperature; for the steel example, it is heated in a liquid salt bath at about I300F for 2 minutes and air cooled to ambient temperature. This treatment causes the reversion of essentially all of the martensite to austenite; the steel had the following mechanical properties: 104,000 psi yield strength, l36.200 psi tensile strength, 36% elongation and 28 R hardness. During tensile testing some of the austenite transformed to martensite indicating that the M,, has been increased to above ambient temperature.

The steel can be again cooled to a temperature below M,- to convert the austenitic structure to martensite and again heated to a temperature above the austenite reversion temperature but below the austenite recrystallization temperature. This procedure of cycling between martensite and austenite can be repeated until the desired mechanical properties are obtained. After five complete cycles, the reverted austenite state for the example had the following mechanical properties: 162,000 psi yield strength. 193,000 tensile strength, 30% elongation and 42% R, hardness. During tensile testing a substantial portion of the austenite transforms to martensite indicating that the M has been increased as compared to after the first reversion cycle.

Alloy Selection For the initial experimentation related to this invention, the austenite to martensite transformation was obtained by cooling in liquid nitrogen (320F). One of the strongest of the prior art TRIP steels has a composition of about 9% Cr8% Ni4% Mo2% Mn2% Si0.3% C. Unfortunately this alloy was unsatisfactory since its M,- is less than 320F. The nominal TRIP steel composition of 24% Ni4% Mo0.3% C, which is the B alloy reported upon by Zackay, et al. was found to be useable for the initial experimentation. Two different carbon contents of this nominal composition were used as shown below:

Designation Composition Bl Fe-24.2% Ni-4.l% Mo-.29% C B2 Fe-24.5% Ni-4.l% Mew-.227: C

cause the M, proved to be too low (less than 452F).

Accordingly, an operable compositional range for the alloying ingredients of this invention was l9.9-24.5%"

Nickel, 2.0l4.l% Mo, up to 1.85 Cr, up to 0.25% Mn, up to 0.24% Si, and 0.220.36% Carbon.

Hardness Measurements Samples about 1 X l X l/4 inches thick of alloys B1 and B2 were austenitized at 2050F and water quenched. The hardness was about 80 R R) in this condition. After'cooling to -196C and returning to room temperature, the hardness of the martensite and austenite structure was 31 /z 33 R,. The samples were then immersed in a salt pot at various temperatures from 900 to 1700F for 2 minutes and air cooled to room temperature. The hardness and structure of both alloys after this heat treatment are shown in FIG. 1. The temperature for complete austenite reversion was between ll00 and l200F, and it was apparent that complete reversion would occur under conditions such that theaustenite is appreciably harder than in the as-quenched condition. The Bl alloy that had been reverted at l200F was cycled through another 320F and l200F sequence, and it was noted that a duplex austenite/martensite structure then existed. The reversion temperature was evidently deemed dependent upon some structural feature that varied with continual cycling.

Another Bl sample, that had been originally reverted at l300F, was processed through four additional 320F and l300F cycles, and this sample was always entirely austenitic after each l300F treatment. FIG. 2 shows the hardness ofthe Bl sample that was processed through five austenite and martensite (320F) and austenite (l300F) cycles after each step in the process. After the fifth reversion, the hardness of the austenite was 42 /& R and it appeared that the hardness was leveling off near this value I During the course of this research, hardness readings were also made in 0.060 inch thick samples of both alloys cycled between 320F and l300F. The hardness of the'austenite is shown as a function of the number of cycles in FIG."3 for both thicknesses (0.060 inch and inch) of alloy B1, and the thinner samples are seen to be softer than the A inch thick samples. The same feature was also found' in 0.060 inch and inch thick samples of alloy B2 processed through five cycles. Although an explanation for this behavior was not established, a number of variables were investigated; included in the factors that experimentally did not account for this hardness/thickness behavior are grain size, original austenitizing (solution) temperature, cooling rate down to 320F, time at 320F, and time at l300F.

Metallography The structural characteristic of the as-quenched (from 2050F) material from which all the previously mentioned results were obtained is shown in FIG. 4. The grain size is about ASTM 0. This material was taken from an as-forged bar where forging temperatures up to 2250F were used; this accounts for the extremely large starting grain size.

6 After a single treatment at 320F, the'duplex martensite/austenite structure appeared as shown in FIG.

5. The martensite needles reached a lengththat appeared' to be limited by the austenite grain size. After a 2 minute reversion at l300F, the reverted austenite appeared as a ghost" structure of the prior martensite as shown in FIG. 6; these ghost areas are seen to etch darker than the original martensite. Upon cooling to 320F of a once reverted sample. the martensite that formed appeared to be primarily in the virgin austenite rather than the reverted austenite. An example of the martensite, reverted austenite and virgin austenite structures is shown in FIG. 7. It might be anticipated that additional cycling would tend to convert the remaining virgin austenite to reverted austenite. This was confirmed; FIG. 8 shows the structure after the fifth reversion, and it is estimated that about 95% to 98% of the material is now reverted austenite.

In an attempt to determine whether the type of structure seen in FIG. 8 would undergo recrystallization, the sample that had been cycled through five reversions at l300F was heat treated at 1900F for minutes. The resulting structure is shown in FIG. 9; in general, a considerable amount of grain refinement occurred (see FIG. 4 for the initial grain size note difference in magnifications). The initial grain boundaries are still evident, but it is not clear whether these are real or ghost boundaries.

X -Ray Line Broadening Measurements -(2'050F) condition, after a single reversion at l300F and after the fifth reversion at l300F. The intensity vs. diffraction angle (29) for the three conditions is shown in FIG. 10. In the as-quenched condition, the separation of the K d 1 and K d, 2 peaks are readily apparent, but the broadening in both the other conditions eliminates the separation. The intensity (26) b ehaviors of the reverted austenites are characteristic of that normally observed in cold worked metals.

The amount of line broadening (width of the reflection at one-half the maximum intensity) was measured in the three cases, and values of 025, l.20, and l-.62 were obtained for the as-quenched, the single reverted austenite, and the five cycle reverted austenite. respectively. In addition to the increase in the broadening with cycling, FIG. 10 shows that the diffraction angle corresponding to maximum intensity increases from about l27.75 (26) for the as-quenched condition to about l28.65 after the fifth reversion. This increase in 29 indicates that a residual compressive stress is created by the cyclical processing.

Mechanical Properties Tensile testing was done after processing of both inch and 0.060 inch thick material of alloy B1. As previously noted, the inch thick samples achieved somewhat higher hardness values than the 0.060 inch thick samples.

A 0.3 inch thick section of alloy B1 was homogenized at l900F and water quenched; separate tests indicated that this lower solution temperature produced equivalent hardness values through five reversion cycles. Indiand all martensite to austenite transformations were done by heating for 2 minutes in a salt bath at l3()0F and air cooling. The yield strength, tensile strength. and elongation obtained with various samples are shown in FIG. 14. A single reversion is seen to increase the yield 5 strength by about a factor of 2 A; without any significant loss in elongation. The primary reason for this is that during testing of the once reverted sample. the martensite transformation was induced and this promotes high elongation values for the existing flow stresses. The strain induced martensite transformation did not occur during testing of as-quenched samples. A second reversion cycle is seen to further increase the yield and tensile strength at an equivalent elongation. After the fifth reversion cycle. the yield strength has been increased by a factor of about four and the tensile strength nearly doubled with only a small loss in elongation.

Stress-strain curves for some of these samples are shown in FIG. 11. The curves for the once and twice reverted samples are characteristic of TRlP steels thermomechanically processed -7 prior to testing. For samples processed through five reversions, the stressstrain curve shows a yield point, L'u'ders straining and work hardening stages. all of which are found in the stronger TRlP steels.

Samples of the B1 alloy 0.060 inch thick were solution treated at l900F and processed through five reversion cycles as a function of reversion temperatures from ll50 to l350F. After the fifth reversion. the samples reverted at ll50 and l200F consisted of a duplex martensite-austenite structure and those reverted at l250F to I350F were entirely 997!) austenitic. The yield strength. tensile strength. and elongation are shown in FIG. 12 as a function of reversion temperature. One of the most significant features of FIG. 12 is the decrease in yield strength at reversiontemperatures where the martensite-austenite is not completed. The presence of untempered martensite also results in lower elongation values butdoes not appear to affect the tensile strength.

A comparison of the properties obtained in the 0.060 inch and inch thick samples given equivalent processing of five cyclical reversions at 1300F shows the The reason for this effect was not established in this initial investigation.

The normal thermomechanical processing of TRIP steels accomplishes two essential features. First, the processing must strengthen the austenite; without this processing. yield strengths of only 40-50 ksi exist. Second. the processing must raise the M,, above the testing temperature (intended service temperature). The M,, is the temperature below which the austenite and martensite transformation can be induced during plastic deformation. If a perfectly processed TRlP steel is tested above the M,, temperature, the elongation is reduced to only [/5 to [H0 of what can be found at temperatures below the M In the initial investigation. it is apparent that both austenite strengthening and the raising of the M above ambient has been established by thermal processing alone. The yield strength was increased from 42.6 ksi to I62 ksi (after five reversion cycles). Most of this strengthening is presumed to be the result of martensite (shear) transformation and is retained during the reverse transformation to austenite. Precipitation of carbides (M0 or Fe) may have contributed a minor amount to the strengthening. The comparison between carbide precipitation during thermomechanical processing and thermal processing is not known.

The change in the M,, is also thought to be related to the martensite shear transformation; the higher internal strain energy favors the austenite to martensite transformation. thus increasing the M,,. Again, however, precipitation of carbides could also have contributed some effect to the increase in M by changing the chemistry of the matrix.

The results clearly show that thermal processing can be used to condition" TRlP steels. Using the results of Zackey. et al., a comparison of the mechanical properties following thermomechanical processing (80% reduction at 930F) and thermal processing for the same alloy is shown below:

Thermomcchanical g (80% reduction at 930F) I64 I76 4| following. Thermal (5 cycles between 320F d I300F l Yield Tensile 21 62 30 Sample Thickness Strength Strength Elongation (z'cydes between ii 0.060" thick samples l42 ksi I84 ksi 27% F) /i" thick samples 162 ksi 193 ksi 30% 50 It is obvious that a process consisting of 5 cycles between 320F and 1300F can provide the physical characteristics of a thermomechanical processing of reduction at 930F.

TABLE I Alloy Num- Ni Mo Cr Mn Si Co C Fe bet I 24.2 4.1 .29 Balance 2 24.5 4.1 .22

4 3.6 3.85 .36 5 24.2 3.97 25 .24 .30 6 2l.8 3.79 .96 .28 7 20.2 3.86 L78 .32 s 24.0 2.91 .94 .30 9 23.6 2.01 l.8l .30 l0 2| .9 2.96 .96 .30

ll l9.9 2.03 1.85 .30

TABLE l-continued Alloy Num- Ni Mo Cr 7 Mn Si Co C Fe ber TABLE I1 MECHANICAL PROPERTIES AFTER VARIOUS THERMAL PROCESSES.

Structure Structure Yield Tensile Prior Prior to After Strength.* Strength. Elongation Thermal Processing Test Test ksi ksi 7r As-Quenched austenite austenite 42.6 98.3 38% As-Quenched Plus martensite & martensite & austenite 1 19.6 155.9 13

Liquid Nitrogen austenite Reverted at 1300F austenite austenite & martensite 104.4 136.2 36

2 Reversion Cycles austenite austenite & martensite 136.4 164.9 36% 3 Reversion Cycles austenite austenite 8: martensite 144.3 163.6 33

5 Reversion Cycles austenite austenite 8c martensite 162.0 193 We claim as our invention:

l. The process of producing articles from a steel of high-strength and elongation having a metallurgical structure essentially comprised of reverted austenite, by controlled heat treatment, the process comprising:

heating a steel blank to a temperature at which said blank is completely austenitic, the chemistry of said blank consisting essentially of O.220.36% carbon, 19.924.5% Ni, 2.04.l% Mo and the balance essentially iron, and selected to produce initial thermal martensite start temperature (M and deformation martensite start temperature (Md), both below ambient and an austenite reversion temperature below the temperature at which austenite recrystallizes, cooling the steel blank to produce a predominantly austenitic structure, subjecting this cooled blank to a temperature sufficiently low to transform a substantial portion of the austenite to martensite having a first level of strength, heating this martensitic blank to a temperature adequate to cause a substantial portion of the martensite to revert to austenite, but at a temperature below that at which the reverted austenite recrystallizes, said reverted austenite having a first level of elongation, cooling this reverted austenite blank to cause its 'structure to become at least partially martensitic and repeating this cycle of reverted austenite-martensite-reverted austenite until both the M,, is increased above said ambient temperature and the blank defines an article having a strength level greater than said first strength level and an elongation level at least 80% of said first elongation level.

2. The process recited in claim 1, in which the blank is additionally subject to strain below the final M thereby inducing the transformation of the reverted austenite steel to martensitic steel, whereby the strength of the strained steel is increased above the strength level resulting from said thermal cycling.

3. The process recited in claim 1, in which the blank is additionally tempered in the reverted austenite.

4. A method of enhancing the strength and elongation levels of a steel through thermal cycling, comprisa. preparing a steel blank having a chemistry consisting essentially of 0.22'-O.36% carbon, 19.9-24.5% Ni, 20-41% Mo and the balance essentially iron, and selected to provide an alloyed composition with an M,- and an M temperature below ambient and an austenite reversion temperature below the austenite recrystallization temperature.

b. austenitizing said blank and then cooling to below said M, temperature,

c. subjecting said blank to at least one heating and cooling cycle wherein the blank is heated to above the austenite reversion temperature but below the austenite recrystallization temperature so that said blank is predominantly and increasingly constituted of reverted austenite, the blank is then cooled to below said M, temperature, and

d. in 'the final cooling step, limiting said cooling to above the M,- temperature.

5. The process of producing a steel of high strength and high elongation having a metallurgical structure essentially comprised of reverted austenite, by controlled heat-treatment the steel being particularly characterized in that it has a thermal martensite starting temperature (M well below ambient temperature and the lowest anticipated service temperature, and a deformation martensite start temperature (M also below the ambient temperature, and an austenite reversion temperature below the austenite recrystallization temperature, said steel consisting essentially of 0.220.36% carbon, l9.924.57c Ni, 2.04.l% No and the balance essentially iron being subjected to the following heat treatment:

a. heating the steel to a temperature at which the steel is completely austenitic,

b. cooling said steel to a temperature below said (M,.)

to provide a predominantly martensitic steel structure,

c. heating said predominantly martensitic steel to a temperature above said austenite reversion temperature but below said austenite recrystallization temperature to provide a structure predominantly of reverted austenite, and

1 l 12 coollrlg this Steel h preciomlnamly reverted provide said steel with a strength exceeding that of the austemte Structure bemg rammed repeatmg 531d steel as obtained from the first cooling step (b) and an heat-treatment cycle of transformation from auselongation at least 80% of the elongation of the steel as temtic via martensltlc to reverted austemtic struc- 5 obtained from the first step (c).

ture. 6. The process as in claim 5, in which said repetition The Process as m clalm m Much the reverslon of said heat treatment is carried out to provide an intemperature 18 at about 3 F- crease in the M,, over the starting value, and also to 

1. THE PROCESS OF PRODUCING ARTICLES FROM A STEEL OF HIGHSTRENGTH AND ELONGATION HAVING A METALLURGICAL STRUCTURE ESSENTIALLY COMPRISED OF REVERTED AUSTENITE BY CONTROLLED HEAT TREATMENT, THE PROCESS COMPRISING: HEATING A STEEL BLANK TO A TEMPERATURE AT WHICH SAID BLANK IS COMPLETELY AUSTENITIC THE CHEMISTRY OF SAID BLANK CONSISTING ESSENTIALLY OF 0.23-0.36% CARBIN. 19.9-24.5% NI. 2.0-4.1% MO AND THE BALANCE ESSENTIALLY IRON, AND SELECTED TO PRODUCE INITIAL THERMAL MARTENSITE START TEMPERATURE ATURE (M8) AND DEFORMATION MARTENSITE START TEMPERAURE MD) BOTH BELOW AMBIENT AND AN AUSTENITE REVERSION RECRYSTALLIZES COOLING THE STEEL BLANK TO PRODUCE A PRETEMPRATURE BELOW THE TEMPERATURE AT WHICH AUSTENITE DOMINANTLY AUSTENITIC STRUCTURE SUBJECTING LOW TO TRANSFORM A BLANK TO A TEMPERATURE SUFFICIENTLY LOW TO TRANSFORM A SUBSTANTIAL PORTION OF THE AUSTENITE TO MARTENSITE HAVING A FIRST LEVEL OF STRENGTH HEATING THIS MARTENSITIC BLANK TO A TEMPERATURE ADEQUATE TO CAUSE A SUBSTANTIAL PORTION OF THE MARTENSITE TO REVERT TO AUSTENITE BUT AT A TEMPERATURE BELOW THAT AT WHICH THE REVERTED AUSTENITE RECRYSTALLIZES SAID REVERTED AUSTENITE HAVING AT FIRST LEVEL OF ELONGATION COOLING THIS REVERTED AUSTENITE-MARTENSITE AND REPEATING THIS TO BECOME AT LEAST PARTIALLY MARTENSITIC AND REPEATING THIS CYCLE OF REVERTED AUSTENITE-MARTENSITE-REVERTED AUSTENITE UNTL BOTH THE MD IS INCREASED ABOVE SAID AMBIENT TEMPERATURE AND THE BLANK DEFINES AN ARTICLE HAVING A STRENGTH LEVEL GREATER THAN SAID FIRST STRENGTH LEVEL AND AN ELONGATION LVEL AT LAST 80% OF SAID FIRST ELONGATION LEVEL.
 2. THE PROCESS RECITED IN CLAIM 1 IN WHICH THE BLANK IS ADDITIONALLY SUBJECT TO STRAIN BELOW THE FINAL MD THEREBY INDUCING THE TRANSFORMATION OF THE REVERTED AUSTENITE STEEL TO MARTENSITIC STEEL, WHEREBY THE STRENGTH OF THE STRAINED STEEL IS INCREASED ABOVE THE STRENGTH LEVEL RESULTING FROM SAID THERMAL CYCLING.
 3. The process recited in claim 1, in which the blank is additionally tempered in the reverted austenite.
 4. A method of enhancing the strength and elongation levels of a steel through thermal cycling, comprising: a. preparing a steel blank having a chemistry consisting essentially of 0.22-0.36% carbon, 19.9-24.5% Ni, 2.0-4.1% Mo and the balance essentially iron, and selected to provide an alloyed composition with an Ms and an Md temperature below ambient and an austenite reversion temperature below the austenite recrystallization temperature, b. austenitizing said blank and then cooling to below said Ms temperature, c. subjecting said blank to at least one heating and cooling cycle wherein the blank is heated to above the austenite reversion temperature but below the austenite recrystallization temperature so that said blank is predominantly and increasingly constituted of reverted austenite, the blank is then cooled to below said Ms temperature, and d. in the final cooling step, limiting said cooling to above the Ms temperature.
 5. The process of producing a steel of high strength and high elongation having a metallurgical structure essentially comprised of reverted austenite, by controlled heat-treatment the steel being particularly characterized in that it has a thermal martensite starting temperature (Ms ) well below ambient temperature and the lowest anticipated service temperature, and a deformation martensite start temperature (Md), also below the ambient temperature, and an austenite reversion temperature below the austenite recrystallization temperature, said steel consisting essentially of 0.22-0.36% carbon, 19.9-24.5% Ni, 2.0-4.1% No and the balance essentially iron being subjected to the following heat treatment: a. heating the steel to a temperature at which the steel is completely austenitic, b. cooling said steel to a temperature below said (Ms) to provide a predominantly martensitic steel structure, c. heating said predominantly martensitic steel to a temperature above said austenite reversion temperature but below said austenite recrystallization temperature to provide a structure predominantly of reverted austenite, and d. cooling this steel with its predominantly reverted austenite structure being retained, repeating said heat-treatment cycle of transformation from austenitic via martensitic to reverted austenitic structure.
 6. The process as in claim 5, in which said repetition of said heat treatment is carried out to provide an increase in the Md over the starting value, and also to provide said steel with a strength exceeding that of the steel as obtained from the first cooling step (b) and an elongation at least 80% of the elongation of the steel as obtained from the first step (c).
 7. The process as in claim 5, in which the reversion temperature is at about 1300*F. 